In general, bacteria such as Legionella attach to circulating water or the like of an air conditioning unit and along with operation of such equipment, the bacteria are discharged outdoors through an outlet, as a result, the bacteria float in the air. In addition, bacteria such as Legionella proliferate also in the remaining bath water of a home bathtub. Active oxygen, particularly, ozone is a very strongly oxidative substance, and ozonated water containing dissolved ozone is being utilized in a field requiring ozone, for example, for cleaning/sterilization treatment such as sterilization at a food processing or medical site, for water treatment/sterilization of water/sewage or wastewater (e.g., general wastewater, ballast water) containing organic materials such as malonic acid and odorous geosmin, bacteria or the like, or for cleaning in a semiconductor device manufacturing process. As the method for producing ozonated water, a method of producing ozone in water by electrolysis of water is known. Furthermore, a peroxide such as ammonium persulfate is known as a cleaning agent such as resist remover used similarly in the process of producing a semiconductor circuit, and there is used an electrolytic oxidation reaction of sulfuric acid by active oxygen such as ozone produced upon an electrolytic reaction of aqueous sulfuric acid in the production of the peroxide, or an electrolytic reaction directly from sulfuric acid.
As the anode for ozone production, which is used in the electrolysis of water, an electrode obtained by coating lead oxide on a substrate such as valve metal, or a pure platinum (Pt) electrode is used. However, the former has a problem of reducing the electrode life due to delamination during electrolysis or an environmental problem such as elution of a harmful substance adversely affecting the human body, and the latter electrode not only fails in obtaining an adequate electrode activity (oxygen overvoltage) but also is expensive to cause an economical problem. As described in Patent Literatures 1 to 3 recited later, an electrode composed of titanium (Ti) and platinum (Pt) is also known in general, and the Pt-coated Ti may be free from an environmental problem of the lead oxide (PbO2) or an economical problem of the pure platinum material but cannot obtain an adequate electrode activity (oxygen overvoltage).
In addition, there is a problem that a long electrode life cannot be sufficiently obtained. Also, for example, an electrode produced by heat-treating a Pt-coated Ti alloy at 400 to 700° C. and an electrode produced by forming a layer from a mixture of Pt powder and Ti metal or Ti oxide powder have been developed. However, these electrodes have a problem such as low ozone production efficiency and heavy wear.
The material used as the anode for ozone production is a valve metal such as Ti, zirconium (Zr), niobium (Nb) and tantalum (Ta), which is generally employed as the electrode substrate material. Many valve metals are known to have a high oxygen overvoltage and therefore, have an ozone formation potential when used as an anode. However, the surface of an electrode formed of a valve metal is oxidized by electrolysis and the oxide layer becomes thick and serves as an insulator, as a result, the function as an electrode is impaired and the electrode is short-lived. For this reason, it is considered to utilize a thin Ta oxide layer as a dielectric material on the electrode surface (WO 2003/000957). However, even when this electrode is used, there is a problem that although a good ozone production efficiency may be obtained at the initial stage of use, the initial characteristics are not kept long and the electrode life is insufficient.
In order to solve those problems, an electrode where a noble metal layer such as Pt is formed as an intermediate layer on an electrode substrate and sequentially, a valve metal oxide layer (dielectric layer) such as Ti is formed in the surface region has been reported (Patent Literatures 1 to 3). These electrodes have a problem that formation or firing of a thin layer must be repeated a plurality of times so as to form an intermediate layer or a surface layer and the number of working steps is large. Also, there is a problem that the adherence between the noble metal layer and the valve metal oxide layer is not sufficient and furthermore, the electrode surface property at the electrolytic reaction interface is changed during use, failing in maintaining a high electrolytic oxidizing ability for a long time.
As the electrolytic anode used for the production of a persulfuric acid such as ammonium persulfate, for example, a platinum ribbon is used. However, a sufficient oxygen overvoltage is not obtained by an electrolysis treatment using the platinum ribbon and the wear amount of electrode becomes large under harsh electrolysis conditions, giving rise to a problem that a wasted electrode component is mingled as an impurity in the electrolytic solution or a problem that the electrode must be changed frequently.
For solving such a problem, there has been devised a production method of a persulfuric acid-dissolved water, including electrolyzing a sulfate ion-containing aqueous solution by using an electrode to produce a persulfuric acid-dissolved water, where the electrode uses a substrate such as valve metal and contains a less oxidizing metal such as platinum group metal in an intermediate layer and a valve metal oxide-containing surface layer is formed on the electrode surface (JP-A-2007-016303). However, even by this method, a high oxygen overvoltage cannot be continually obtained and not only the electrode life is short but also the persulfuric acid production efficiency is insufficient.
Conventionally, for the anodic electrode used in Cr(VI) plating, lead or a lead alloy has been used because of its high electrolytic oxidizing ability. Such an anode can oxidize Cr(III) to Cr(VI) to properly control the Cr ion concentration but has a problem that, for example, lead chromate is precipitated in a large amount due to anode dissolution during use or a lead compound or lead ion is mingled in the waste liquid.
For solving such a problem, a method of performing plating by using, for the anode, an insoluble electrode using a platinum group metal and an oxide thereof as the main component is known. However, such an anodic electrode is very low in the ability of oxidizing Cr(III) to Cr(VI) as compared with the anodic electrode made of lead or a lead alloy and therefore, has a problem that the Cr ion concentration in the plating bath can be hardly controlled.
As a measure to solve this problem, there has been devised a method where an insoluble electrode obtained by coating a platinum group metal on a Ti-containing metal substrate is used and an additive such as silver nitrate or silver oxide is added to the plating solution to thereby control the Cr ion concentration in the Cr(VI) plating (JP-A-2006-131987). However, even when this method is used, the Cr ion concentration cannot be sufficiently controlled, and a problem such as control of the additive amount or contamination of the chromium plating film by the additive exists.